The questions isn't asking 'suggest why a lower estimate and higher estimate were made', it's asking about how carbon dioxide and methane contribute to temperature increases. This would be an appropriate answer:

Carbon dioxide and methane are both greenhouse gases. An increased volume of these greenhouse gases in the atmosphere result in an enhanced greenhouse effect because these gases stop infrared radiation escaping from the atmosphere. This results in a mean surface temperature increase.

(Original post by rhianna1)
The questions isn't asking 'suggest why a lower estimate and higher estimate were made', it's asking about how carbon dioxide and methane contribute to temperature increases. This would be an appropriate answer:

Carbon dioxide and methane are both greenhouse gases. An increased volume of these greenhouse gases in the atmosphere result in an enhanced greenhouse effect because these gases stop infrared radiation escaping from the atmosphere. This results in a mean surface temperature increase.